Use of an Anode For Elimination or Reduction of Microbial Impurities in Liquids.

ABSTRACT

The present invention relates to the use of an anode suitable for use in inter alia a reactor for elimination or reduction of microbial impurities from liquids, such as inter alia waste water and water intended for human or animal consumption. The anode comprises an expanded metal plate, preferably titanium, covered with a non-corrosive metal layer, preferably a platinum layer. The surface of the anode is endowed with dents, which enhances the electrochemical effect between the anode and the corresponding cathode and thereby enhances the microbial effect and at the same time reduces the energy required to obtain an efficient kill of the microorganisms,

FIELD OF THE INVENTION

The present invention relates to the technical field of electrochemicalelimination or reduction of microbial impurities of liquids. The liquidstreated may inter alia include waste water and water intended for humanconsumption.

BACKGROUND OF THE INVENTION

Conventional methods of elimination or reduction of microbial impuritiesin liquids, such as waste water and water intended for human and animalconsumption, typically included use of chemicals, biochemical treatment,sedimentation, distillation, filtration, electrochemical devices or thelike.

Electrochemical devices comprise one or more anodes and cathodes thattypically are arranged in order to allow liquids to pass therebetween.Moreover, various types of structural and compositional surfaces of theelectrodes are possible in order to generate a variety of differentreactions in the liquid that passes between the two electrodes. At theanode, halides may be oxidised to their corresponding halogen, mostcommonly chlorine, via dimerisation of halogen radicals and water may beoxidised to dioxygen and protons. At the cathode dioxygen may be reducedto hydrogen peroxide, and water to hydrogen and hydroxyl ions. Chlorine,chlorine radicals, hydrogen peroxide and ozone may all have a biocidaleffect on the bacteria content in the treated liquid.

There are several problems associated with the use and generation ofchlorine in water treatment, particularly due to its potential negativeeffects on the environmental as well as the legal limits of the chlorinelevel present in water intended for human and animal consumption.Examples of undesirable environmental effects of the use of chlorine arethat it reacts with nitrogenous compounds resulting in chloramines,which are poor biocides with unpleasant odours. Furthermore, chlorine isreactive with other organic materials and may result in environmentallyharmful, carcinogenic and/or teratogenic compounds such as chloroform orchloroalkanes as well as reacting with naturally occurring phenoliccompounds to form chlorinated compounds. In waste water treatment,chlorination must be followed by process of laborious and potentiallynoxious dechlorination using sulphur dioxide or an equivalent chemicalthereof in order to comply with discharge chlorine levels.

However, in recent years the use of chlorine has been increasinglydiscouraged and limited. For example, the German drinking waterdirective (based on EU Council Directive 98/83/EC of November 1998 onthe quality of water intended for human consumption) limits the presenceof chlorine in drinking water to 0.5 mg/l. Additionally, in large partsof the food industry high concentrations of chlorine in water that comein direct contact with food products are also prohibited. Usually waterof “drinking quality” is considered acceptable in that context and isgenerally the quality of water specified for use in many processes ine.g. food factories.

In order to provide the desired reduction in water in the number ofbacteria capable of creating colonies, including pathogenic bacteria, itis often necessary to use concentrations of chlorine that are markedlyhigher than the allowable limit in drinking water. In the EuropeanHygienic Engineering and Design Group guidelines “Shafe and HygienicWater Treatment in Food Factories” it is stated that levels of chlorineup to 1000 ppm can be required to control bacteria, i.e. maintaining thenumber of bacteria below the colony forming level. This obviouslycomplicates even further the utilization of chlorine in water cleansingsystems.

Another major problem with electrochemical cleansing of e.g. waste waterand water intended for human and animal consumption, has been theeconomically unfavourable energy requirements of the cleansing systems.In recent years considerable efforts to reduce the energy costs of saidsystems, e.g. via optimisation of the electrodes utilized, has beenmade.

Prior art describing similar systems for purification of liquidincludes.

U.S. Pat. No. 4,316,787 discloses an anode comprising a laminated bodyof a platinum group metal foil bonded to a niobium or tantalum layer,which in turn is bonded to a titanium substrate. The anode is operableat a voltage above 20 volts and a watt density above 100 watts persquare inch surface. Hence, this anode consists of three metals whereasthe anode of the present invention only consists of two metals,preferably titanium and platinum, and furthermore is operable at lowervoltages at 10-15 volts.

US2003/0164308 discloses a method and an apparatus to obtain drinkingwater from waste water, based on the use of an electrolytic cell forminga part of a dynamic flow system which operates at a relatively lowvoltage (20-200 volts, 1-6 amperes) and at very high flow rates. Theanode is in contrast to the anode according to the presentinvention—formed from iron, stainless steel, carbon or copper.

U.S. Pat. No. 4,290,873 discloses an electrode mesh comprised oftitanium or tantalum as the base-material and covered with platinum. Theplatinum may be mechanically clad to the titanium or tantalum substrate,or the platinum may be plated electrolytically onto the substrate. Theshape and surface of the anode together with the thickness of thetitanium layer of one hundred microinches (2.54 μm) is different fromthe anode according to the present invention.

U.S. Pat. No. 3,616,355 discloses an anode, which comprises a laminatedbody of a platinum metal foil on a substrate or backing of a metal suchas titanium, tantalum or niobium. The bonding of said materials is beingeffected by a highly localised pressure and thermoelectric heat. Incontrast to this, the anode of the present invention consists ofexpanded metal and is also endowed with dents in the platinum surface.

DE19625254 discloses an anode of expanded titanium covered with a layerof platinum. The anode is characterised by the way the two layers areattached, which is different from the anode of the present invention.Furthermore, the anode is not endowed with dents on the surface.

DE2223240 discloses an anode comprising titanium and platinum. However,this anode is not made from expanded metal and is also not endowed withdents on the surface. Furthermore, the anode is made with the purpose ofapplying nitration during electrolysis.

DE3823760 discloses an anode comprising an expanded metal titanium platecovered with platinum. However, the thickness of the platinum layer ismore than three times thicker and thus more expensive than the anode ofthe present invention. Furthermore, the anode is not endowed with dentsin the platinum surface.

SUMMARY OF THE INVENTION

The present invention relates to a novel use of an anode—which comprisesa plate of expanded base-material, preferably titanium, endowed at thesurface with dents having a diameter of preferably 10-40 μm in an amountof preferably 50-500 dents per square millimetre—In a method forelimination or reduction of microbial impurities in liquids. Thestructure of the anode is best defined by the process at which it isproduced. The process involves subjecting the anode plate—insuccession—to degreasing, acid washing, preferably with nitric acid(HNO₃), glass-blowing and electrolysis in order to cover the anode platewith a layer of pure platinum. The structure of the anode obtained bythis particular sequence of process-steps has surprisingly shown toproduce a considerably higher biocidal effect in relation to the energyrequired to operate the system i.e. the current and voltage required,compared to the anodes known in the art.

DESCRIPTION OF THE INVENTION

The present invention relates to the use of a specifically preparedanode in a method of electrochemical elimination or reduction ofmicrobial impurities of liquids, such as waste water and water intendedfor human consumption. The elimination or reduction of microbialimpurities via the anode according to the present invention is based onthe biocidal effect, which is achieved from the produced chloride-basedand oxygen-based compounds. The anode comprises a plate of an expandedbase-material, preferably consisting of titanium, covered by ananti-corrosive material, preferably platinum. The surface of the anodeis endowed with dents, which enhances the electrochemical effect betweenthe anode and its corresponding cathode and thereby enhances thebiocidal effect of the microorganisms while—at the same time—reducingthe energy required to obtain an efficient biocidal effect.

More specifically, the utilisation of extendable base-material with welldefined dents provides a natural turbulence when the liquid passesthrough its surface, which consequently enhances the formation ofbiocidal chlorine as the individual water molecules has to be in closeproximity of the surface of the anode in order to perform the requiredchemical reactions. Furthermore, the well-defined dents result inchanged flow-conditions and/or larger surface areas, which furtherenhances the formation of biocidal chlorine. As the chemical reactionstakes place at the close proximity of the surface of the anode a largesurface area as well as increased waterflow, i.e. via turbulence or thelike, over the anode inevitably increases the resulting biocidal effect.

Furthermore, the present invention relates to a novel use of anspecifically prepared anode suitable for a method of elimination orreduction of microbial impurities in liquids, such as waste water andwater intended for human or animal consumption, while—at the sametime—maintaining drinking water quality and avoiding excess use ofchemicals. Maintenance of drinking water quality is defined herein, asthe presence of chlorine in drinking water is limited to 0.5 mg/l orbelow (in accordance with the German drinking water directive based onEU Council Directive 98/83/EC of November 1998 on the quality of waterintended for human consumption). The water resulting from thedisinfection process can be used directly for human consumption or usedin a variety of industrial processes in which such a high quality isrequired.

Advantages Over Prior Art

The anode according to the present invention provides a comparativelyhigh biocidal effect in relation to the energy-requirements of thesystem. At the same time the chlorine content produced by the system isbelow the levels allowed or in accepted International drinking waterdirectives, e.g. the above mentioned German drinking water directive.This improved functionality is provided by the unique structure of theanode according to the present invention, which is best defined by thespecific process of which it is produced, i.e. by degreasing, acidwashing, preferably with nitric acid (HNO₃), glass-blowing andelectrolysis in order to cover the anode plate with a layer of pureplatinum.

The extent of the applicability of the invention appears from thefollowing description. It should, however, be understood that thedetailed description and the specific examples are merely included toillustrate the preferred embodiments and that various alterations andmodifications within the scope of protection will be obvious to personsskilled in the art on the basis of the detailed description.

Applications of the Anode of the Present Invention

In preferred embodiments the anode of the present invention isapplicable to waste water streams such as waste water from, for example,sewage plants, electroplating operations, food processing plants, fabricdye facilities, and the like. The present invention is also useful fortreating water streams for producing purified drinking water. Thepresent invention is particularly applicable to oil-water emulsions. Theterms waste stream or liquid stream, as used herein, refers to suchwaste water streams and other liquid streams, including some non-aqueousliquid streams.

In a further embodiment the anode according to the present invention canalso be used in systems for on-site treatment of inter aliadomestic-type waste, such as ships, trains, aircrafts and off-shoredrilling platforms. At such locations, the waste typically flows througha biological or fermentation unit on board, and then into a holdingtank. When the effluent in the holding tank reaches a certain level, itis pumped through a sterilising unit where the effluent is sterilised,usually with sodium or calcium hypochlorite. The effluent is then pumpedoverboard. Such treatment is usually costly and requires the use oflarge and heavy, space consuming equipment.

In an even further preferred embodiment the anode of the presentinvention is applicable to water supply plants including plants fortreatment of ground water, surface water, desalted water, rainwater anddrinking water from devices such as drinking water automat machines.

In an even further preferred embodiment the anode of the presentinvention is applicable to water utilized in the manufacturing of soapand cosmetics.

In an even further preferred embodiment the anode of the presentinvention is applicable to water utilized in the production of plastic.

In an even further preferred embodiment the anode of the presentinvention is applicable to water utilized in the food industry,including water utilized in the preparation of spices, fish/shellfish,chicken/poultry, pork/beef, margarine, confectionery, dairy products,beer/mineral water, vegetables, candy/chewing gum, animal feed and Inwater used in cold or refrigerated storage facilities.

In an even further preferred embodiment the anode of the presentinvention is applicable to water from district heating station, bathwater, domestic hot water plant such as jacuzzis and pools, hospitalsand old people's home.

In an even further preferred embodiment the anode of the presentinvention is applicable to water from printing houses, retail trade,fountain basins, metal industry, paint and lacquer industry, households,gardening, such as water from liquid manure, and biotech industry, suchas water from the fermentation and pharmaceutical industry.

The anode is suitable for water treatment using various apparatus forinstance such apparatuses as described in U.S. Pat. No. 6,309,519,EP0997437 and U.S. Pat. No. 6,652,733.

DETAILED DESCRIPTION OF THE INVENTION

The biocidal effect shown by an anode according to the present inventiondepends on the magnitude of the flow over the anode—or through thereactor equipped with said anode—as well as the density of the currenton the anodes. Therefore, if the water is led slowly through the reactorand/or a high current density is applied, a higher biocidal effect isobtained. Hence, it is a matter of optimisation to find the suitableflow rate and current density in a given application in order to achievea satisfying biocidal effect as well as maintaining a high capacity.Therefore the number of reactors, the flow and the current density mustbe corrected according to the given conditions.

Preparation of the Anode According to the Present Invention

In order to provide the specific structure of the anode plate accordingto the present invention, the plate is—in the following order—subjectedto:

-   -   degreasing    -   treatment with suitable acid, preferably nitric acid (HNO₃) or        oxalic acid (H₂C₂O₄) (acidic washing)    -   glass-blowing, preferably with glass-particles or—beads of        preferably 75-150 μm in diameter    -   platinum plating (e.g. via conventional electrolysis)

The dents produced at the surface of the anode have diameters of 10-40μm and are present in an amount of 50-500 dents per square millimetre.

The Electrochemical Mechanisms of the Anode According to the PresentInvention

At the anode according to the present invention, hydroxide ions (OH⁻)naturally contained in the water donates electrons to the cathode andare thus converted to oxygen gas. This gas is subsequently eliminatedfrom the water. Hence, the concentration of hydrogen tons (H⁺) in thewater increases rendering the water acidic. Also at the anode chlorideions (Cl⁻) contained in the water donate electrons to the cathode andbecome chlorine gas (Cl₂). The chlorine gas dissolves in the acidicwater and is converted to hypochlorous acid (HOCl).

The cathode donates—at the close proximity of the cathode—electrons tothe hydrogen ions (I) contained in the water to become hydrogen gas,which subsequently is eliminated from the water. Also at the cathode,sodium ions (Na⁺) as well as hydroxide ions (OH⁻)—if present in thewater—are bonded and sodium hydroxide is formed.

The bacteria are killed—i.e. the bioddal effect—by chemically derivedoxidation occurring when the water is electrolysed. The anode accordingto the present invention produces both chlorine-based and oxygen-basedoxidants, which are formed according to the following reactions:

2Cl⁻→Cl₂+2e ⁻

Cl₂+H₂O→HOCl+HCl

HOCl+H₂O→OCl⁻+H⁺

2H₂O+2e ⁻→H₂+2OH⁻

2H₂O→2H₂+O₂

O₂→O⁻

H₂O+O→H₂O₂

Of these, dichlorine (Cl₂), hypochlorous acid (HOCl), ozone (O₃),hydrochloric acid (HCl), hydrogen peroxide (H₂O₂), oxychloride (OCl⁻)and hydroxide (OH⁻) have proved to be hazardous to microorganisms.

EXAMPLES Example 1 Killing Efficiency and Time of Treatment

An example of the present invention will now be described with referenceto the accompanying drawing, in which:

FIG. 1 is a schematic perspective diagram of an example water treatmentdevice according to the present invention.

Referring to FIG. 1, a water treatment device according to an example ofthe present invention comprises anodes 1 with an expandablebase-material and cathodes 2, which are held in non-conductingstructures (not shown) to maintain a constant distance between theelectrodes. The electrodes are connected to a DC power supply. Theelectrodes 1 and 2 are inserted into the water to be treated.

A reactor being 12 cm wide, 7 cm high and 40 cm long (=3,360 cm³=3.36liter) comprising numerous of the anodes as described in example 2 aswell as cathodes of stainless steel was used to disinfect 500 litre ofwater per hour at 10-15 Volts. Prior to treatment, the microorganismscontained in the water corresponded to 10,000 CFU/ml. After the flowthrough the reactor, the content of microorganisms was reduced to 1CFU/ml and contained a chloride concentration of less than 0.5 mg/lchlorine. The process is continues and the time for the water to passthrough the reactor was approximately 7 seconds, i.e. a “pass throughtime” of 3.36 liter/7 seconds, i.e. 0.48 liter/second.

Example 2 Method for Preparing an Anode

An anode of 10×33 cm having a thickness of 1.5 mm was made of a plate ofexpanded metal titanium with the following characteristics:

Standard: DIN Standard 791 Type F

Mesh Dimensions 6×3×1.0×1.0 mm (mesh-length × mesh-with × rib-with ×rib-thickness)

Material: Titanium Gr. 1: Type 3.7025 DIN 17860

The plate was degreased and treated with oxalic acid. It was thenglassblown with glass particles having sizes of 75-150 μm. By use ofelectrolysis the plate was covered with pure platinum by a conventionalprocess. The thickness of the resulting platinum layer was 1.5±0.3 μm.The dents at the surface of the anode had diameters of 10-40 μm and waspresent in an amount of 50-500 dents per square millimetre.

Example 3 Comparative Study of the Chlorine Production Versus DifferentMethods of Producing the Anode

In order to evaluate possible effects on the chlorine production as aconsequence of different methods of producing the anode, the followingthree anode plates were tested under the same electrical conditions,i.e. at current intensities between 11-11.8V and at a voltage of 10A.

Chlorine liberated in close vicinity of Anode plate produced by (infollowing order) the anode (mg/l) Acidic washing with nitric acid →Glass blowing 0.136 Glass blowing → Acidic washing with oxalic acid0.060 Acidic washing with oxalic acid → Glass blowing 0.082

As can be seen the specific sequence of acidic washing followed by glassblowing was superior in relation to the liberation/formation of chlorinecompared to the sequence of glass blowing followed by acidic washing,Secondly, the use of nitric acid (HNO₃) in the acidic washing appearedto be superior over oxalic acid in the relation to the subsequentliberation/formation of chlorine.

It also appeared that the concentrations of liberated chlorine was wellbelow the acceptable 0.5 mg/l limit according to the previouslymentioned drinking water directive.

As part of the biocidal effect of the anode is a direct consequence ofchlorine-based compositions naturally occurring in the surroundingwater, application of sodium chloride (NaCl⁻) might be beneficial. Forexample if the chlorine content in the liquid of a certain applicationis very low or non-existing, the kill-effect will also be lowered due tothe reduced production of the chlorine compositions. In such asituation, it may optionally be necessary to apply NaCl⁻.

1. Use of an anode comprising a plate of expandable base-materialsubjected to: degreasing acidic washing glass-blowing plating withanti-corrosive material in a process for elimination or reduction ofmicrobial impurities in liquids.
 2. Use of an anode according to claim 1wherein the expandable base-material is resistant to corrosion and atwhich platinum or similar metals can attach
 3. Use of an anode accordingto claim 1 wherein platinum or similar metals can attach to theexpandable base-material.
 4. Use of an anode according to claim 1wherein the expandable base-material is titanium or similar metals. 5.Use of an anode according to claim 1 wherein the anode is characterisedin comprising indentations or dents, the diameter of said indentationsor dents being between 10-40 μm.
 6. Use of an anode according to claim 5wherein the number of indentations or dents per square millimetre of theplate is between 50-500.
 7. Use of an anode according to claim 1 whereinsaid acidic washing is carried out with nitric acid
 8. Use of an anodeaccording to claim 1 wherein said glass-blowing is applied using glassparticles or glass beads of 75-150 μm in diameter.
 9. Use of an anodeaccording to claim 1 wherein the plating layer comprises pure platinumwith a thickness 1.5±0.3 μm.
 10. Use of an anode according to a claim 1wherein the anode has a cylindrical geometrical shape.
 11. Use of ananode according to claim 1 wherein multiple the plates are arranged in asandwich-like or similar fashion.
 12. Use of an anode according to claim1 wherein the distance between the plates are 0.5-2.0 mm.
 13. Use of ananode according to claim 1 wherein said liquid is water, including wastewater from, for example, sewage plants, electroplating operations, foodprocessing plants, fabric dye facilities, and the like.
 14. Use of ananode according to any of claim 1 wherein said liquid is subjected to afiltrating pre-treatment.
 15. Use of an anode according to claim 1wherein said liquid is oil-water emulsions.
 16. Use of an anodeaccording to claim 1 wherein said anode is applied as offshore treatmentof said liquid.
 17. Use of an anode according to claim 1 wherein saidliquid is intended for drinking water or other purposes in which waterof drinking water quality is required.